A method of processing the output signal of a measuring transducer is disclosed. A force-measuring device, in particular a balance, which is equipped with a measuring transducer whose output signal is being processed according to the method of processing the output signal of a measuring transducer.
The measuring accuracy of a force-measuring device, for example a balance based on the principle of electromagnetic force compensation or strain gauge technology (see reference [1], “Bauen Sie Ihre Qualität auf solidem Grund!” (Build your Quality on Solid Ground!), company publication, Mettler Toledo GmbH, January 2001, pages 14-15) is determined by numerous influence factors which are described in [2], “Wägefibel” (Weighing Primer), Mettler Toledo GmbH, April 2001. Particularly undesirable are disturbances caused by mechanical factors such as vibrations or shocks, which is the reason why already balances using analog signal processing were equipped with filters serving to remove disturbances in the signal.
Reference [3], CH 673 529 A5 discloses a balance with an active low-pass filter which is designed to suppress signal disturbances in the form of unwanted AC components that are superimposed on the DC signal produced by the weighing cell and sent through a signal line to an analog/digital converter. The unwanted signal components are tapped off the signal line at the signal output terminal of the weighing cell, their phase is changed by 180° by an inverter, and the inverted AC disturbance component is fed back into the signal lead at the input terminal of the analog/digital converter, wherein the signal lead itself has an ohmic resistance between the aforementioned tap-off node and feed-back node. Consequently, the disturbing signal components are canceled by matching signal components of opposite phase.
Reference [4], DE 10024986 A1, describes an electronic weighing transducer with a digital signal-processing unit in which a DC component in the output signal of the weighing transducer is determined by means of a filter with a low-pass characteristic, and the weighing result is determined from the filtered DC component. At the same time, a shock- and vibration-dependent signal is determined and dependent on the latter the DC component of the measuring signal is altered.
According to reference [4], the foregoing concept avoids drawbacks that occur with the solution disclosed in [5], U.S. Pat. No. 5,665,941. According to [5], the time constant of the low-pass filter in a differential dosage-dispensing balance is changed dependent on the disturbance component in the signal. In this arrangement, the time constant of the low-pass filter is lengthened in the case of large disturbances in order to achieve a stronger filtering effect. However, as stated in [4], the concept described in reference [5] causes the weighing transducer to react sluggishly to changes while increasing the reproducibility of the measurements only to an insignificant extent. If the time constant is selected too large, this will further result in a long settling time for transient oscillations that accompany changes in the weighing load.
Further methods as well as balances in which said methods can be used are described in references [6], US 2004/0088342 A1, and [7], U.S. Pat. No. 6,271,484 B1, wherein the signals produced by the measuring transducer are processed by means of variable digital filters.
With the method described in [6] the characteristic of the filter being used can be adapted individually to the oscillatory properties of the measuring system that is being controlled by the method. The damping of the filter can therefore be increased to any desired degree within a selected frequency range.
According to the method described in [7], a test is made to determine whether the amplitude of the vibration-related signal disturbances lie within a permissible range. If this is not the case, the filter characteristic is changed until the signal disturbances are within the permissible range again.
In particular the last-described method requires a high amount of computing power and, due to the time constant of the servo loop, hardly allows a fast enough adaptation to rapid amplitude changes of vibrations and oscillations as soon as they occur.
The oscillations of rapidly changing amplitude which occur with changes of the weighing load are, however, corrected only to an insufficient extent with the existing solutions, some of which are very complex and expensive, if there is a simultaneous requirement to realize a short settling time for transient oscillations after a change in the weighing load. Because in the previously practiced solutions, the aim was in most cases to achieve a compromise between the strongest damping of oscillatory disturbances combined with the longest response time on the one hand and the weakest damping of oscillatory disturbances combined with the shortest response time on the other hand, none of the results achieved so far in regard to response time as well as in regard to the filtering of disturbances in the signal have been good enough to meet the most stringent requirements.
The disclosures of references [1] to [8] are hereby incorporated by reference herein in their entireties.